Yellow-Cedar Decline

• In 2023, more than 20,000 acres of active yellow-cedar decline were mapped during aerial detection survey, a two-fold increase from last year. 
• Active decline was most concentrated on the central Tongass National Forest, broken down as follows:
  • Zarembo, Wrangell and Etolin Islands and the nearby coastal mainland (6,500 acres)
  • Kuiu, Mitkof, and Kupreanof Islands, as well as Cape Fanshaw and the Cleveland Peninsula (7,000 acres)
  • Baranof and southern Chichagof Islands, especially along Salisbury Sound (3,200 acres)
  • Gravina and Annette Islands (500 acres)
  • Prince of Wales Island (2,200 acres, surveys limited by weather)
  • Northern Chichagof Island (600 acres).
• Yellow-cedar mortality was first detected in Glacier Bay National Park in 2021. This year we mapped 60 acres near La Perouse Glacier, Finger Glacier, and Icy Point. We hope to ground check this new northern margin of yellow-cedar decline, though access is difficult. 
• More than 710,000 acres of yellow-cedar decline have been mapped across Southeast Alaska. See the table of cumulative yellow-cedar decline, with the total acreage of aerially mapped decline by ownership type, geographic area, and National Forest Ranger District. 
• We continue to monitor yellow-cedar decline in young-growth stands managed for timber. In 2023, intensifying mortality and crown discoloration symptoms of decline were noted in several stands on Zarembo and Wrangell Islands through aerial detection and ground-based plots. In established plots, 4% of yellow-cedar was dead and 45% had crown symptoms of yellow-cedar decline.​
• Our partners also conducted important work related to yellow-cedar in 2023.
  • Benjamin Gaglioti and Daniel Mann (University of Alaska Fairbanks) collected tree cores from healthy, unhealthy, and dead yellow-cedar at Fick Cove on Chichagof Island. Study goals are to establish when tree growth impacts become evident in tree ring records and to identify drought indicators in wood cells associated with fine root loss.
  • Forest Service Ecologists Sean Cahoon (PNW Research Station) and Kate Mohatt (Chugach National Forest) launched a project to document growth, mortality, and population characteristics of yellow-cedar at the northern edge of its range in Prince William Sound. Results from the study could offer insights into possible refugia conditions that promote yellow-cedar growth in a changing climate. 

Young-growth yellow-cedar decline is an emerging issue, particularly where soils are wet or shallow. The problem was first observed in young-growth forests on Zarembo Island in 2012; before that, decline had only been observed in old-growth forests. To facilitate young-growth yellow-cedar decline monitoring, we compiled a database of 338 managed stands on the Tongass National Forest with yellow-cedar, but more remain to be added. Alongside the database, low-altitude aerial imagery and aerial detection surveys are used to identify stands with discolored tree crowns and suspected decline, which are then inspected on the ground. In 2023, intensifying mortality and crown discoloration symptoms of decline were noted in several stands on Zarembo and Wrangell Islands through aerial detection and ground-based plots. To date, decline has been ground-verified in 33 young-growth stands on Zarembo, Kupreanof, Wrangell, Mitkof, and Prince of Wales Islands. Affected stands are typically 27- to 45-years-old, precommercial thinned between 2004 and 2012, and occur on sites with south to southwest aspects and wet or shallow soil.

In 2018, we installed 41 permanent plots in the five most severely affected stands on Zarembo, Kupreanof, and Wrangell Islands to quantify the impacts of yellow-cedar decline. The mortality rate for yellow-cedar was just 2% overall, yet far exceeded negligible mortality of associated tree species. The three units on Zarembo Island were remeasured in 2023. Of 1,143 yellow-cedar trees assessed, 45% had crown discoloration symptoms and 4% were dead (3-6% per stand), demonstrating that the problem is continuing.

Now that yellow-cedar decline is known to occur in young-growth stands, hydrology and soil temperature should be considered before precommercial thinning and other management activities occur, particularly in stands that are not expected to retain persistent snowpack in decades to come. Yellow-cedar planting sites should be carefully selected with both snowpack and deeper rooting depth in mind, promoting yellow-cedar where it is expected to thrive long-term.

Bioevaluation reports are available from work on young-growth yellow-cedar decline on Zarembo Island (Mulvey et al. 2013, updated 2015) and Kupreanof Island (Mulvey et al. 2015) and from our effort to quantify decline impacts in the most severely affected young-growth stands on Kupreanof, Wrangell and Zarembo Islands (Mulvey et al. 2019).

Yellow-cedar crop trees on Kupreanof  and Zarembo Islands with discolored, thinning crowns.

A map of young-growth stands with yellow-cedar and the incidence and severity of yellow-cedar decline.

Yellow-cedar decline functions as a classic forest decline and has become a leading example of the impact of climate change on a forest ecosystem. The term forest decline refers to situations in which a complex of interacting abiotic and biotic factors leads to widespread tree death, usually affecting one tree species or genus over an extended period of time. It can be difficult to determine the mechanism of decline, and the causes of many forest declines worldwide remain unresolved. Hennon et al. (2012) provide a detailed summary of the interdisciplinary research approach at multiple spatial and temporal scales that unraveled the complex causes of yellow-cedar decline. 

Temporal patterns include the timing of yellow-cedar forest development and favorable climate (based on tree age), long-term linkages between climate patterns and pulses of decline, and fine-scale study of air and surface soil temperatures in research plots to identify temperature thresholds and mortality events. Most of the trees that have died within the last century, and continue to die, regenerated during the Little Ice Age (1400-1850). Heavy snow accumulation is thought to have occurred during this period, giving yellow-cedar a competitive advantage on low-elevation sites in Southeast Alaska. Trees on these sites are now susceptible to exposure-freezing injury under warmer climate conditions. An abnormal rate of yellow-cedar mortality began around 1880, accelerated in the 1970s and 1980s, and continues today. These dates coincide with the end of the Little Ice Age and a warm period in the Pacific Decadal Oscillation. On a finer temporal scale, recent analysis of 20^th century weather station data from Southeast Alaska documented increased temperatures and reduced snowpack in late winter months, in combination with the persistence of freezing weather events in spring (Beier et al. 2008). From the time crown symptoms appear, it takes 10 to 15 years for trees to die, making it difficult to associate observations from aerial surveys to weather events in particular years. Mortality has subsided somewhat in the last three decades.

Spatial patterns of decline range from landscape level (latitude and elevation, patterns of snow persistence), to site level (mortality concentrated where hydrology or bedrock restricts rooting depth and snowpack does not persist in early-spring), to tissue level (sensitivity of yellow-cedar to freezing injury in spring). Recent mortality is most dramatic on the outer and southern coast of Chichagof Island (Peril Strait) and at higher elevations, indicating an apparent northward and upward spread that is consistent with the climate patterns believed to trigger mortality. At the southern extent of decline in Alaska (55-56° N), mortality occurs at relatively higher elevations, while farther north, decline is restricted to relatively lower elevations. Yellow-cedar forests along the coast of Glacier Bay and in Prince William Sound appear healthy, though small pockets of mortality have recently been detected in Glacier Bay National Park. These northerly parts of the range are presumably protected by deeper and more persistent snowpack. In 2004, a collaborative aerial survey with the British Columbia Forest Service found that yellow-cedar decline extended at least 100 miles south into British Columbia. Since that time, continued aerial mapping around Prince Rupert and areas farther south have confirmed more than 120,000 acres of yellow-cedar decline in BC. 

Over 710,000 acres of decline have been mapped in Southeast Alaska through aerial detection survey since surveys began in the 1980s, with extensive mortality occurring in a wide band from the Ketchikan area to western Chichagof and Baranof Islands. The cumulative estimate includes forests mapped in the 1980s that were killed since the turn of the century. The hypothesis that has emerged is consistent with the observed patterns: conditions on sites with exposed growing conditions and inadequate snowpack in spring are conducive to premature root tissue dehardening, resulting in spring freezing injury to fine roots and gradual tree mortality. The temporal patterns help to explain why yellow-cedar occurs on many sites where it is currently maladapted. Yellow-cedar is protected by persistent snowpack and deeper soils.

Symptoms: Symptoms of yellow-cedar decline include yellow-red-brown foliage discoloration (affecting greater than 15% of the tree crown) and crown dieback, which results from damage to the fine roots. Root-freezing injury often kills individual trees slowly over more than a decade, causing the tree crown to gradually thin and discolor. Some trees remain alive for decades with only 5-10% of the original foliated tree crown, having lost most of the biological and ecological function of a live tree. Affected forests often contain trees at various stages of decline. Research on seasonal cold tolerance of yellow-cedar has demonstrated that yellow-cedar trees are cold-hardy in fall and mid-winter, but are highly susceptible to spring freezing. Yellow-cedar roots are more vulnerable to freezing injury, root more shallowly, and de-harden earlier in the spring than other conifer species in Southeast Alaska (Schaberg et al. 2005).

Associated Damage: Trees weakened by freezing injury may die rapidly if they are attacked and girdled by secondary bark beetles. Comprehensive assessment of pathogens and insects associated with declining trees found that Phloeosinus bark beetles (Phloeosinus cupressi) and Armillaria root disease play only minor roles in yellow-cedar mortality, attacking trees stressed by fine-root freezing injury (Hennon 1986, 1990a). Although it is not possible to see dead fine roots with the naked eye, it is possible to see dark-colored necrotic (dead) tissue moving up from dead coarse roots when the bark around the roots and root collar is removed.

A dark-colored lesion at the root collar of a declining yellow-cedar crop tree with bark removed (left).
Mycelium of the Armillaria root rot fungus beneath the bark of a declining yellow-cedar (right).

Signs of secondary bark beetles, Phloeosinus cupressi, larvae, adult beetle, and galleries.

Ecological Impacts: Yellow-cedar is an economically and culturally important tree. Yellow-cedar decline changes stand structure and composition (Oakes et al. 2014). Snags are created, and succession favors other conifer species. In some stands, where decline has been ongoing for up to a century, understory shrub biomass increases significantly. Nutrient cycling may be altered, especially with large releases of calcium as yellow-cedar trees die. The creation of numerous yellow-cedar snags is probably not particularly beneficial to cavity-nesting animals because its wood resists decay, but may provide branch-nesting and perching habitat. On a regional scale, excessive yellow-cedar mortality may lead to diminished populations (but not extinction). The loss of yellow-cedar in some areas may be balanced by increases in yellow-cedar in others, such as higher elevations and parts of its range to the northwest. Yellow-cedar is preferred deer browse, and deer may significantly reduce regeneration where their populations are high and spring snowpack is insufficient to protect seedlings from early-season browse.

A new study has been published that evaluated the economic returns and ecological impacts of salvage logging on forest succession (Bidlack et al. 2022). They found that small-scale salvage harvest did not impact forest succession or yellow-cedar abundance, and that economic returns were small to moderate but varied by location. Bidlack et al. also completed a final report on this topic in 2019.

Dr. Ben Gaglioti (University of Alaska- Fairbanks) and others used a dendrochronology approach to date when long-dead yellow-cedars had died alongside La Palouse Glacier. They estimated that all but one had been standing for more than a century (Gaglioti et al. 2021). It is unknown whether these very old mortality events at this location were caused by yellow-cedar decline or other factors related to glacial change.​

Salvage recovery of standing dead yellow-cedar trees in declining forests can help produce valuable wood products and offset harvests in healthy yellow-cedar forests. Cooperative studies on mill-recovery and wood properties of yellow-cedar snags that have been dead for varying lengths of time (Kelsey et al. 2005) showed that all wood properties are maintained for the first 30 years after death. At that point, bark is sloughed off, the outer rind of sapwood (~0.6" thick) is decayed, and heartwood chemistry gradually changes. Decay resistance is altered somewhat due to these chemistry changes, and mill-recovery and wood grades are reduced modestly over the next 50 years. Remarkably, wood strength properties of snags are the same as that of live trees, even after 80 years. Localized wood decay at the root collar finally causes snags to fall about 80 to 100 years after tree death. The large acreage of dead yellow-cedar, the high value of its wood, and its long-term retention of wood properties suggest promising opportunities for salvage.

Appearance, characteristics, and mean time-since-death for the five dead tree (snag) classes of yellow-cedar (Hennon et al. 1990b).

See the annotated bibliography of yellow-cedar (Hennon and Harris 1997), and Literature Cited sections of Hennon et al. (2016, 2012).

Click here for full list of yellow-cedar pubs (links to full text)

Hennon, P. E ; Harris, A. S. 1997. Annotated bibliography of Chamaecyparis nootkatensis. Gen. Tech. Rep. PNW-GTR-413. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 119 p. Available here.

Hennon, P. E.; D'Amore, D. V.; Schaberg, P. G.; Wittwer, D. T.; Shanley, C. S. 2012. Shifting climate, altered niche, and a dynamic conservation strategy for yellow-cedar in the North Pacific coastal rainforest. BioScience. 62: 147-158. Available here.

Hennon, P. E.; McKenzie, C. M.; D'Amore, D. V.; Wittwer, D. T.; Mulvey, R. L.; Lamb, M. S.; Biles, F. E.; Cronn, R. C. 2016. A climate adaptation strategy for conservation and management of yellow-cedar in Alaska. Gen. Tech. Rep. PNW-GTR-917. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 382 p. Available here.